Using Essential Oils to Control Moss and Liverwort in Containers

Nabil Khadduri

Nabil Khadduri is Grower, Meridian Seed Orchard (formerly Nursery Scientist, Webster Nursery), Washington Department of Natural Resources, PO Box 47017, Olympia, WA 98504; Tel: 360.789.9483; E-mail: [email protected]

Khadduri N. 2011. Using essential oils to control moss and liverwort in containers. In: Riley LE, Haase DL, Pinto JR, technical coordinators. National Proceedings: Forest and Conservation Nursery Associa- tions—2010. Proc. RMRS-P-65. Fort Collins, CO: USDA Forest Service, Rocky Mountain Research Station: 133-138. Available at: http://www.fs.fed.us/rm/pubs/rmrs_p065.html

Abstract: Liverwort and moss are economically significant weeds across a range of US container production sites, including forest seedling greenhouse culture in the Pacific Northwest. We have demonstrated the effectiveness of essential oils, or distilled plant extracts, in controlling liverwort and moss container weeds over three seasons of trials. When applied at the appropriate concentration and volumes, essential oils act as an effective contact herbicide. There is little or no residual, and under heavy pressure, liverwort and moss can re-establish within 2 weeks of knockdown. The applicator must pay close attention to method and application rates to successfully control weeds while avoiding crop damage, such as leaf or needle burn. Leaf wilt following the first drying cycle has occasionally been observed, with root damage suspected but not confirmed. This paper summarizes integrated pest management practices to limit the invasion and spread of liverwort and moss in containers, details post- emergent trial results of directed-spray and over-the-crop herbicide applications, suggests guidelines for species and stocktype stages, and provides a checklist of procedures when applying essential oils.

Keywords: essential oils, liverwort, moss

Liverwort and Moss in Container Nurseries______A national survey of container and nursery growers and extension specialists ranked liverwort as the number two weed in horticultural production (IR-4 2009). This survey also identified liverwort, moss, and algae as the third largest “group” of weeds in container production, trailing only broadleaf plants and sedges. Liverwort and moss reduce crop growth and value by outcompeting crops for water and nutrients and by providing habitat and encouraging the development of fungus gnat populations. In the Pacific Northwest (and throughout much of the US), Marchantia liverwort (Marchantia polymorpha) is the liverwort best adapted to container culture, while silver-thread moss (Bryum argenteum) is the most likely moss to infest greenhouses. Liverwort and moss are lower plants, meaning they lack a vascular system. They are the cockroaches of the plant world, with liverwort thought to be the first group of plants to make the transition from blue-green algae to successful land coloni- zation. Liverwort and moss have several reproductive strategies, including asexual propagation through simple division as well as the unique ability of liverwort to develop gemmae cups. These cups fill with gemmae, small plant fragments, which can splash up to a meter away from the mother plant when hit at the right angle with a drop of water (Figure 1). Liverwort and moss also reproduce sexually, developing spore-releasing capsules.

USDA Forest Service Proceedings RMRS-P-65. 2011 133 Using Essential Oils to Control Moss and Liverwort in Containers Khadduri

180 °F) for up to a minute, that is, just short of “warping” the blocks. Additional pre-crop sanitation includes removal of any existing liverwort and moss adjacent to greenhouses. Bak- ing soda, liberally sprinkled on liverwort in non-crop areas (complete coverage is not necessary), acts as an effective contact herbicide for initial eradication. Combinations of weed barrier and mulch, weed barrier alone, or pre-emergent herbicides such as flumioxazin or dimethenamid-P may be applied to limit liverwort and moss growth.

Sanitation During Propagation Once a greenhouse crop cycle begins, the best liverwort and moss prevention strategy is to kill any spores that float into the greenhouse before they make it past the germination stage. Applying ZeroTol™ (BioSafe Systems, Glastonbury, CT) (hydrogen dioxide, peroxyacetic acid, and stabilizers) at 1:450 as a post-rinse following each irrigation can successfully reduce spore loads (Thompson 2008). Another spore-killing strategy is to apply a constant ultra-low dose chlorine or copper treatment to kill spores through an in-line system.

Water Management Greenhouse cultural practices are paramount to liverwort Figure 1. Marchantia liverwort (Marchantia polymorpha) can develop and moss control, with particular emphasis on water manage- gemmae cups. The gemmae fragments within can splash up to 1 m ment; these weeds thrive in the presence of moisture. Choose (3 ft) away from original plant. mulches, such as screened grit, that are standardized in size, smooth-surfaced, and therefore drain well. Uniformly cover soil medium at time of propagation, as exposed surfaces will be become infested quicker. Try to be a “dry” grower. To the extent possible, water by plant needs rather than human convenience. Up-front costs spent on determining when a crop actually needs irrigation Pre-crop Sanitation will be rewarded with less water usage, lower pest pres- Spore-release of liverwort and moss is widespread, so the sure, and ultimately a higher quality crop. Once a decision primary goal of a nursery manager should be to kill spores to irrigate has been made, consider expediting crop drying with a spreader that breaks down water-surface tension, between crops. Pre-crop sanitation may include the use of ® ® pressure washing to remove bulk debris and follow-up with a such as Sylgard 309 or R-11 (Wilbur-Ellis, Fresno, CA). chemical treatment; any chemical treatment is always more This practice is especially called for in late-day irrigations effective with thorough pre-washing. Chemical treatments or irrigations in overcast weather. Use horizontal airflow that are commonly used to reduce spore loads of liverwort, fans in opposing patterns to break up boundary layers and moss, and algae are quaternary ammonium chlorides, sold monitor and reduce excessive relative humidity where pos- as products such as GreenShield® (Whitmire Micro-Gen sible. Again, liverwort and moss need moisture to thrive, in Research Laboratories, St Louis, MO), Physan 20™ (Maril almost all cases more than the crop you are actually trying Products Incorporated, Tustin, CA), and Triathlon® (OHP to grow. Starve them of excess moisture as a basic preven- Incorporated, Mainland, PA). Other chemical treatments tion technique. include hydrogen dioxide, chlorine dioxide, and chlorine bleach (Smith 2007). Forest seedlings are commonly grown in Styroblock™ Nutrient Management containers (Beaver Plastics, Acheson, Alberta); along with Liverwort and moss love nutrients and, like many plants, other container styles, these may be a vector for liverwort thrive in nitrogen and phosphorus-rich environments. and moss spore contamination from one crop to the next. Growers may have a tendency to apply excessive amounts Probably the most effective treatment to sterilize blocks, of fertilizer (“luxury consumption”) to push seedlings to particularly for older blocks where old plant debris is very size simply because stock that does not meet minimum difficult to physically remove, is steam cleaning. Steam morphological specifications is less marketable. Seedling cleaning setups are expensive, but alternatives exist. At nutrient analysis can help a grower differentiate between Washington Department of Natural Resources Webster hidden hunger, optimal growth, and luxury consumption, Nursery (Olympia, WA), we use a hot-water dip tank to thereby targeting nutrient applications at levels that do not clean blocks. We run temperatures at 77 to 82 °C (170 to excessively stimulate liverwort and moss growth.

134 USDA Forest Service Proceedings RMRS-P-65. 2011 Khadduri Using Essential Oils to Control Moss and Liverwort in Containers

Since liverwort and moss are shallow-rooted (not really to occur. A grower can take advantage of the limited or non- having roots at all, but rather root-like structures called existent cuticle of liverwort and moss to burn these plants rhizoids), they thrive in environments where nutrients are while waxier surfaces, such as conifer needles, are relatively surface applied. Limiting surface-applied nutrient applica- protected. tion generally means decreasing fertigation, the practice Growers in the Pacific Northwest first experimented of applying fertilizer through the irrigation system. When with Cinnamate (cinnamon oil) (Acros Organics, Fairlawn, a grower fertigates, they are applying nutrients to the soil NJ), a product labeled for two-spotted spider mite control surface, feeding liverwort and moss where they live. An al- that turned out to control liverwort and moss as well. Due ternative nutrient-delivery system is to focus at least partly to factory safety issues, Cinnamate is no longer produced. on incorporation of controlled-release fertilizers (CRF) in the Sporatec™ (Brandt Consolidated, Springfield, IL; formerly soil medium, where the majority of nutrients are located sold as Sporan by EcoSMART Technologies Incorporated, safely away from the soil surface. Use CRF formulations that Franklin, TN) is a product that consists of rosemary, clove, cover the duration of the seedling’s life in the container, as and thyme oil. The product is primarily labeled as a fun- top-dressing of CRF returns a grower to a situation where gicide, but is now labeled for liverwort and moss as well. liverwort and moss are fed where they live, that is, on the Sporatec™ is a FIFRA 25(b) product, meaning it is exempt soil surface. Recent research, however, indicates that iron from EPA registration. There is no re-entry interval and no sulfate in CRF form top-dressed on containers has a strong pre-harvest interval. Even so, essential oils like Sporatec™ inhibitory effect on liverwort, with reduced concern over are concentrated oils and should be handled carefully. They nutrient toxicity due to the slow release of the product (Prine can irritate skin, and are particularly troublesome to contact 2010). lens wearers. If you spray near harvest time, I strongly recommend that handlers wear gloves. Post-Emergent Options, or Lack Thereof All the best management practices in the world may not Trials With Essential Oils prevent moss and liverwort outbreaks, particularly when The Stuff Actually Works!—During the past several cloudy, cool weather seems to work against you. Hand- years, we have tested a fair number of products with wide- weeding is an option, but that can damage the crop through ranging and astounding claims. Growers like to call these disturbance of shallow roots or soil loss . Due to the tendency products snake oils, and Sporatec™ certainly struck us as of liverwort to fragment, it sometimes feels like weeding only distilled snake oil at first blush. Nonetheless, we first tested spreads the problem. the product on a liverwort and moss-infested crop of western Terracyte® (BioSafe Systems, East Hartfort, CT) (sodium redcedar (Thuja plicata) seedlings grown in Styroblock™ 2A carbonate peroxy-hydrate) is a granular product labeled for (313A) containers (40 cm3 [2.4 in3]) in the fall of 2008. We post-emergent control of liverwort and moss. In the presence conducted a replicated test at 0, 8, 15.5, and 31 ml/l (0, 1, of water, the granules convert to 34% hydrogen peroxide, 2, and 4 oz/gal) rates and, with each rate, included 0.03% strong enough to burn any foliage it contacts. While the spreader-sticker (Sylgard® 309) and 0.45% Hasten® penetrant product may be applied safely on larger containers where (Wilbur-Ellis, Fresno, CA) based on general label recom- foliar contact can be avoided, the granules tend to hang up in mendations to include these types of adjuvants to improve any sort of denser foliage. Dense foliage also tends to reduce efficacy. The label encouraged thorough coverage, and we the uniformity of application, which diminishes uniformity applied at a rate of 2.3 l/m2 (70 oz/10 ft2). While the 15.5 ml/l of liverwort and moss control. (2 oz/gal) rate showed some tip burning of the redcedar, from Mogeton (Certis, Amesbury,Wiltshire, UK) or Gentry which it recovered, and the 31 ml/l (4 oz/gal) rate severely (Chemtura Corporation, Middlebury, CT) (quinoclamine) is burned foliage, the 8 ml/l (1 oz/gal) rate showed no damage a post-emergent originally used to reduce moss in rice pad- to the crop (Figure 2). Spraying 8 ml/l (1 oz/gal) not only dies. Researchers have demonstrated excellent control over did not damage the redcedar, but successfully controlled a range of species and stocktypes with considerable residual, liverwort and, to a lesser extent, moss (Figures 3 and 4). but the parent company withdrew the product from a long We recorded 91% liverwort control and 75% moss control EPA registration process in early 2008. 9 days after treatment. We followed this trial one week later with a second rate comparison over western redcedar, this time at 0, 4, 8, 12 ml/l Essential Oils—An Alternative (0, 0.5, 1.0 and 1.5 oz/gal) rates. We substituted Syl-tac® (Wilbur-Ellis, Fresno, CA), a combination of ­Sylgard® 309 Means of Control______and Hasten®; by keeping the Sylgard® rate the same, we Essential oils are distilled plant extracts. Some essential decreased the rate of Hasten® penetrant in this combination oils have insecticidal, fungicidal, bactericidal, herbicidal product to one-quarter the rate of the first trial. Even though properties, or all of the above. Modes of action are poorly the same volumes of product were applied in fairly similar understood, but ancient Egyptians used thyme oil (with environmental conditions, control was not as thorough. Even the phenolic compound thymol) to preserve mummies from the 12 ml/l (1.5 oz gal) Sporatec™ rate in this second trial fungal and bacterial attack. For weeds such as liverwort controlled only 81% and 62% liverwort and moss, respectively, and moss, essential oils are thought to simply act as a con- 7 days after treatment (not shown). Since these were not tact herbicide. Thorough coverage of the intended target is head-to-head trials, we can only infer that the lower rate of necessary for proper membrane disruption and weed death penetrant led to the weaker control.

USDA Forest Service Proceedings RMRS-P-65. 2011 135 Using Essential Oils to Control Moss and Liverwort in Containers Khadduri

Oregon State University Test—Ed Peachey, an exten- sion agent from Oregon State University, tested over-the-crop Sporatec™ applications at a nursery in Independence, OR, in 2009. The nursery had 4-l (1-gal) pots of arborvitae (Thuja occidentalis) and Oregon grape (Mahonia aquifolium) coming out of dormancy in spring with thick liverwort infestations. Sporatec™ was applied at rates of 8 and 12 ml/l (1 and 1.5 oz/gal), with and without various adjuvant combina- tions, including the combination and rates from our first trial. Applications were made in succession on 20 April and 24 June 2009. He found greater than 85% liverwort control up to 9 weeks following application at both rates, with some minor phytotoxicity to Oregon grape foliage. There did not appear to be increased performance with addition of any adjuvant combination, but there was a trend of less phytotoxicity when adjuvants were included. Franki Wai-Ki Lam of Brandt, Incorporated believes the spreader may have decreased crop phytoxicity by breaking down the surface tension of the oil droplets, thus lowering their tendency to burn broadleaf foliage. Tests at Webster Nursery—In 2009, liverwort and moss growth was poor, but we did have some decent growth of liverwort and moss in some larger 4-l (1-gal) stock by fall. In early October 2009, we tested direct sprays on liverwort and moss beneath a range of species, including western hemlock (Tsuga heterophylla), Sitka spruce (Picea sitchensis), Douglas-fir (Pseudotsuga menziesii), western white pine (Pinus monticola), ponderosa pine (Pinus ponderosa), noble fir (Abies procera), grand fir (A. grandis), western redcedar, and Port-orford cedar (Chamaecyparis lawsoniana). We Figure 2. One replication of Sporatec™ applied (from top to bottom) at found no phytotoxicity when direct spraying rates as high 0, 8, 15.5, and 31 ml/l (0, 1, 2, and 4 oz/gal) rates at a volume of 2.3 l/ as 15.5 ml/l (2 oz/gal) Sporatec™. While superficial spray m2 (70 oz/10 ft2). Photo is 9 days after treatment. coverage on conifer foliage did not result in any burn, needles tended to be quite waxy by this time of year. Roots were also suberizing by this time. Liverwort and moss were success- fully knocked back, with 95% and 88% control, respectively, 3 weeks after treatment, and 93% and 76% control 6 weeks after treatment.

Figure 4. At 9 days after treatment for the 8 ml/l (1oz/gal) treatment, we Figure 3. Comparison of healthy liverwort (untreated) with severely recorded 91% liverwort control and 75% moss control. Crop phytotoxicity damaged liverwort treated at 8 ml/l (1 oz/gal). was observed at 15.5 ml/l (2 oz/gal) rates and above.

136 USDA Forest Service Proceedings RMRS-P-65. 2011 Khadduri Using Essential Oils to Control Moss and Liverwort in Containers

Irrigation Boom Trials—All trials to this point involved Fine tuning of rates meant that 50% over-application might some sort of hand-held spray apparatus. The next step was to mean crop damage, where 50% under-application would chemigate the product through an irrigation boom, allowing result in no control. To prove to ourselves that we were ac- for improved speed and accuracy. We also wanted to further curately applying product downstream, we collected boom test over-the-crop spraying, particularly over early- to mid- samples both between trials and within a trial (near begin- season conifer “liners.” For these trials, we used a Dosatron® ning and near end of trial) and sent these off for analysis at injector (Dosatron International, Clearwater, FL), set at the the British Columbia lab where the product was developed. strongest rate of 1:50. We further diluted this baseline 2% Lab analysis showed that our system was accurate to within setting by making a pre-mix concentrate of one part oil to +/- 15%, which is important when applying an herbicide over several parts water (Figure 5). This allowed us to constantly plants. agitate the pre-concentrate, ideally creating a more uniform downstream application by minimizing settling of the prod- uct in the irrigation line. We did not include any adjuvants. Suggested Guidelines______After a disastrous first trial where we over-applied volume Consider these a starting point for rates to test in your of product, several additional trials allowed us to refine nursery. Always test a small sample first. rates and volumes to the point where we were getting up to 90% control of liverwort and 75% control of moss, with no phytotoxicity to sensitive species like western hemlock Over-The-Crop Sporatec™ Applications in the middle of its growing season. Unfortunately, these reduced rates only temporarily knocked back the weeds for Based on our trials, the following is a suggested guideline 10 to 14 days, and further trials are needed to see whether for over-the-crop Sporatec™ applications that are applied repeat applications on a shorter timeline can better control with the intention of killing only the liverwort and moss: liverwort and moss in midseason crops. 1. For early to mid-season applications, apply 4.7 ml/l water (0.6 oz/gal water; 0.47%) Sporatec™ with a volume of 4.9 l/m2 (150 oz/10 ft2) or 5.8 ml/l (0.75 oz/gal; 0.59%) in a volume of 3.3 l/m2 (100 oz/10 ft2). 2. For late-season over-the-crop sprays, apply 8 ml Sporatec™/l water (1.0 oz/gal water; 0.78%) in a volume of 3.3 to 4.9 l/m2 (100 to 150 oz/10 ft2). 3. We noted a range of crop resistance, from toughest to most sensitive: ponderosa pine>true firs>interior Douglas-fir>coastal Douglas-fir> western redcedar> western hemlock>western larch (Larix occidentalis). 4. Use extreme caution when treating broadleaves. We noted at least some level of phytotoxicity on all broad- leaves we tested, including red alder (Alnus rubra), black cottonwood (Populus trichocarpa), and creeping Oregon grape. If you do apply over the top of broad- leaves, consider pre- and post-wetting the foliage.

Directed-Spray Sporatec™ Applications Season-long at 8 ml Sporatec™/l water (1.0 oz/gal water). Where moss is present, consider using up to 1.2 ml/l (0.16 oz/ gal) Syl-tac®, as this may improve control. Late-season at 12 ml Sporatec™/l water (1.5 oz/gal water). Again, consider using 1.2 ml/l (0.16 oz/gal) Syl-tac® where moss is present. Do not apply this rate to containers less than 98 cm3 (6 in3) or to plants that have not hardened off. Season-long at 12 ml Sporatec™/l water (1.5 oz/gal water) in the second season of a container crop. One important note is to figure out the minimum volume of product you need for the stage of liverwort and moss in your crop and apply only that amount. It is relatively easy to over-apply when direct spraying. When going after a nasty infestation, remember that more volume equals more active Figure 5. For the boom irrigation trials, we started with a Dosatron® ingredient! injector setting of 1:50. We further diluted one part oil to several parts water, allowing us to pre-agitate the product for better downstream uniformity.

USDA Forest Service Proceedings RMRS-P-65. 2011 137 Using Essential Oils to Control Moss and Liverwort in Containers Khadduri

Checklist for Success______Acknowledgments______When applying Sporatec™, consider the following checklist: I thank Diane Haase, Western Nursery Specialist, for her sponsorship of USDA Forest Service matching grant 2009- 1. Soften water to hardness <100 ppm. The label says DG-11062765-020; Debbie McLean of Webster ­Nursery 200 ppm, but the original developer suggests 100 ppm for assistance with boom trials; Wilbur Ellis Company, is a more appropriate cutoff. Auburn, WA, branch for product donation; and Brandt 2. Adjust water pH to 7.0 or lower. Consolidated, Springfield, IL, for covering lab testing 3. Try to apply when temperatures are greater than costs. 16 °C (60 °F) and less than 29 °C (85 °F). 4. Remove contact lenses (preferably before application). References______5. Maintain constant agitation throughout. Settling of oil IR-4. 2009. Ornamental horticulture program grower and exten- will lead to uneven application rates. sion survey summary—US respondents. URL: http://ir4.rutgers. edu/Ornamental/20082009SurveyResultsFinal.pdf (accessed 21 6. A post-application water rinse 5 minutes after spraying July 2010). may reduce phytotoxicity in sensitive species, particu- Prine J. 2010. Personal communication. Salem (OR): Sungro Hor- larly broadleaves. ticulture, Fertilizer Sales Representative. Smith T. 2007. Cleaning and disinfecting the greenhouse. UMass Extension Fact Sheet. Amherst (MA): University of Massachu- setts, Floriculture Program, Department of Plant, Soils and Final Disclaimer______Insect Sciences. URL: http://www.umass.edu/umext/floriculture/ fact_sheets/greenhouse_management/ghsanitz.html (accessed Many plants were harmed in the testing of this product! 21 July 2010). Always conduct small trials first; use extreme caution when Thompson G. 2008. Personal communication. Rochester (WA): testing with broadleaves, and remember that more is not Weyerhaeuser Company, Head Grower, retired. always better!

The content of this paper reflects the views of the authors, who are responsible for the facts and accuracy of the information presented herein.

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